In view of its intended uses, the wall of a container (or a portion thereof) useful for the preparation of rehydrated solute solutions, of rehydrated blood products, of nutritional solutions, of solutions for medical purposes or of pure water by osmotically driven filtration, consists generally in a semi-permeable membrane.
Thus, a semi-permeable membrane needs to be strong, relatively inert and capable of separating compounds with different molecular weights. A semi-permeable membrane structure is characterized by its molecular weight cut-off (MWCO) defined by the molecular weight at which 90% of the solute will be prevented from permeating through the membrane. Since the permeability of a membrane with a given molecular weight cut-off is proportional to its thickness, the lower the molecular weight cut-off, the lower the thickness of the membrane must be to maintain practical flux rates. However, the thinner this membrane becomes, the lower its strength. On the other hand, the higher the molecular weight cut-off, the more the phenomenon of dialysis can interfere with osmosis. Moreover, the lower the molecular weight cut-off, the more selective the semi-permeable membrane. There are thus advantages of working with a molecular weight cut-off as low as possible while maximizing flux rate.